Compounds and infrared devices including stoichiometric semiconductor compounds of indium, thallium, and including at least one of arsenic and phosphorus

ABSTRACT

A semiconductor layer of In 1-x  Tl x  Q carried on a substrate forms an infrared device, where Q is selected from the group consisting essentially of As 1-y  P y  and 0&lt;x&lt;1, 0&lt;y&lt;1.

FIELD OF INVENTION

The present invention relates generally to components including indiumand thallium .Iadd.and including at least one of arsenic and phosphorus.Iaddend.and infrared devices including same, and more particularly to ..compounds of In_(1-x) Tl_(x) Q, where Q is selected from the groupconsisting essentially of As_(1-y) P_(y), 0<x<1, and 0<y<1,.!..Iadd.stoichiometric semiconductor compounds including indium, thalliumand including at least one of arsenic and phosphorus, .Iaddend.and toinfrared detector and emitter devices including same.

Background Art

The prior art reports staring, i.e., non-scanned, infrared focal planedetector arrays formed on mercury cadmium telluride (. .Hg.sub..7TCD.sub..3 Te.!. .Iadd.Hg.sub..7 Cd.sub..3 Te.Iaddend.), indiumantimonide (InSb) and Pt:Si for midwave infrared (3-5 . .microns.!..Iadd.micrometers.Iaddend.) and mercury cadmium telluride (Hg.sub..78Cd.sub..22 Te) for long wavelength (8-12 . .microns.!..Iadd.micrometers.Iaddend.) purposes. An enormous effort has broughtthese technologies to a mature state where arrays as large as 512 by 512pixels are manufactured with practical yields, at high, but marginallyacceptable, cost. One major failing of the mercury cadmium telluride andindium antimonide based staring focal plane detector arrays is thatindividual elements of the array must be indium bump-bonded to a readoutintegrated circuit on silicon substrates as disclosed, for example, inTimlin et al. U.S. Pat. Nos. 5,227,656 and 5,304,500. Such bump-bondingtechniques limit array sizes and stability. Large numbers of nativedefects also limit performance in mercury cadmium telluride arrays.While the platinum silicon arrays can be potentially integrated onto thesame silicon chip that houses the readout integrated circuit, sucharrays have low quantum efficiencies of approximately 1%, to severelylimit performance.

As further demands are made for increased signal-to-noise ratios athigher, or even. .,.!. non-cryogenic.Iadd., .Iaddend.operatingtemperatures, multi-spectral responses, large arrays of small pixels,longer operational lifetimes and lower production costs, it is dubiousif currently available materials will be satisfactory. In the past,several other materials have been suggested to meet these requirements.These other materials have been built of strained layer superlatticestructures and have been formed as quantum well infraredphotoconductors. These other materials can be built on gallium arsenidesubstrates, enabling them to be fabricated using monolithic integrationwith a readout integrated circuit on the same chip. However, these priorart devices have low quantum efficiencies, less than 10%, and quantumwells having awkward optical arrangements.

It is, accordingly, an object of the present invention to provide a newand improved infrared device.

Another object of the present invention is to provide a new and improvedinfrared detector that can be grown on a substrate that also carries aread-out integrated circuit, to obviate the requirement forbump-bonding.

Another object of the invention is to provide a new and improvedinfrared detector that can be grown on an indium phosphide substrate andis substantially lattice matched to the indium phosphide, to obviate theneed for superlattice structures.

Another object of the invention is to provide a new and improvedinfrared detector compound that can be tailored, with the selection ofappropriate mode fractions, to detecting differing wavelengths is theinfrared spectrum.

Another object of the invention is to provide a new and improvedpseudo-binary alloy compound, particularly adapted for use in infrareddetectors and emitters.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, an infrareddetector or emitter device comprises a substrate and a semiconductorlayer of In_(1-x) Tl_(x) Q carried by the substrate, where Q is selectedfrom the group consisting essentially of As_(1-y) P_(y) and 0<x<1,0≦y≦1. In first and second embodiments of the invention, y=1 and y=0,respectively, so that the layer consists essentially of semiconductorIn_(1-x) Tl_(x) P or In_(1-x) Tl_(x) As. Based on investigations we haveconducted, semiconductor layers of In_(1-x) Tl_(x) P and In_(1-x) Tl_(x)As are rugged and can be epitaxially grown in zinc blende structure forall values of x with lattice constants nearly matching those of InP andInAs, respectively. Such epitaxially grown semiconductor layers ofIn_(1-x) Tl_(x) P and In_(1-x) Tl_(x) As are comparatively free ofnative point defects and have the high mobility and infrared absorptioncharacteristics needed for high performance infrared detector andemitter devices. Based on our investigations, TlP is a semimetal havinga negative band gap (-0.27 eV) analogous to that of mercury telluride(-0.30 eV). The band gap of semiconductor In_(1-x) Tl_(x) P (the bandgaps of semiconductors are always positive) spans the entire long andmidwavelength infrared spectra, for different values of x less than0.76.

InP is desirable for the substrate of In_(1-x) Tl_(x) P because it is ahigh mobility, low dislocation electronic material similar to galliumarsenide, capable of forming ohmic contacts as well as p and n hetero-and homojunctions. In addition, InP has a functional passivant/insulator(SiO₂), enabling it to support high performance read-out integratedcircuit devices. Thus, based on our investigations, semiconductorIn_(1-x) Tl_(x) P epitaxially grown on indium phosphide (InP) cansatisfy all system requirements. Semiconductor In_(1-x) Tl_(x) As isalso a suitable infrared detector with many of the same properties onIn_(1-x) Tl_(x) P. InAs is the substrate best suited for In_(1-x) Tl_(x)As. However, In_(1-x) Tl_(x) P is preferred over In_(1-x) Tl_(x) Asbecause InP is a better substrate than InAs. This is because InP can bedoped, has better device performance and can be lattice-matched betterto semiconductor In_(1-x) Tl_(x) P than semiconductor In_(1-x) Tl_(x) Ascan be matched to InAs. In addition, semiconductor In_(1-x) Tl_(x) P andInP substrates offer the advantage of monolithic integration into areadout integrated circuit chip.

We are aware that attempts have been made to produce long wavelengthinfrared detectors in which layers of indium thallium antimony compounds(In_(1-x) Tl_(x) Sb) are deposited on indium antimonide (InSb). AppliedPhysics Letters, Vol. 62, page 1857 (1993) (. .M. von Schilfgaarde etal..!. .Iadd.M. van Schilfgaarde et al..Iaddend.). A significantdrawback in the use of In_(1-x) Tl_(x) Sb is that is favors a moreclosely packed structure to the zinc blende. However, the greatestdisadvantage in the attempted use of In_(1-x) Tl_(x) Sb, which does notappear to arise in In_(1-x) Tl_(x) P or In_(1-x) TlAs, is that In_(1-x)Tl_(x) Sb cannot be successfully grown on a zinc blende lattice. Thisfailure appears to occur because In_(1-x) Tl_(x) Sb does not have astrong enough thermodynamic force to drive Tl onto the zinc blendelattice.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of specific embodiments thereof,especially when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a side view of a preferred embodiment of the invention;

FIG. 2 are plots of energy band gap vs. values of x for each of In_(1-x)Tl_(x) P, In_(1-x) Tl_(x) As and Hg_(x) Cd_(1-x) Te; and

FIG. 3 are plots of electron mobility vs. temperature for each of In₀.33Tl₀.67 P, In₀.85 Tl₀.15 As and Hg₀.75 Cd₀.22 Te.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference is now made to FIG. 1 of the drawing wherein there isillustrated an infrared device capable of detecting long wavelengthinfrared radiation having a cutoff wavelength of 12 . .microns.!..Iadd.micrometers .Iaddend.and for separately detecting mediumwavelength infrared energy having a cutoff wavelength of 5 . .microns.!..Iadd.micrometers.Iaddend.. The structure illustrated in FIG. 1 can beoperated as a photovoltaic or as a photoconductive detector, dependingon the bias voltages to which electrodes thereof are connected.

In the embodiment of FIG. 1, indium phosphide (InP), semi-metallic bulksubstrate 10 has deposited thereon, preferably by any of liquid phaseepitaxy, metalorganic chemical vapor epitaxy, metalorganic molecularbeam epitaxy or molecular beam epitaxy methods, a rugged semiconductoralloy layer of n type indium thallium phosphide 12 (. .In₁₋ x1Tl_(x1)P.!. .Iadd.In_(1-x1) Tl_(x1) P.Iaddend.) having a zinc blende structure.InP substrate 10 is a good electronic material, i.e. has high carriermobility, low dislocation density (similar to GaAs), has virtually nonative point defects, easily has ohmic contacts and p-n junctions formedthereon, is easily doped, can easily be coated with a functionpassivant/insulator (SiO₂), and electronic devices formed thereon havegood, consistent performance characteristics. The value of x for thecompound of layer 12 is selected such that layer 12 absorbs longwavelength infrared energy preferably having a cutoff wavelength of 12 ..microns.!. .Iadd.micrometers .Iaddend.and is approximately latticematched to InP substrate 10. Based on studies we have performed, latticematching is about 1% and the 12 . .micron.!. .Iadd.micrometers.Iaddend.wavelength cutoff are attained with a value of x₁ =0.67,whereby layer 12 has a bandgap of about 0.1 eV. Layer 12 is doped withsilicon to achieve n type conductivity.

Deposited on layer 12, also by any of the foregoing methods, is afurther indium thallium phosphide semiconductor layer 14 (In_(1-x2)Tl_(x2) P). Based on studies we have conducted, with x₂ =0.57 ruggedlayer 14 absorbs midrange infrared energy having a cutoff wavelength of5 . .microns.!. .Iadd.micrometers .Iaddend.(associated with a bandgap of0.28 eV), while passing the long wavelength infrared energy that isabsorbed by layer 12. The indium thallium phosphide compound of layer 14is doped with any one of zinc, magnesium or beryllium to form a p typelayer, whereby a p-n homojunction is formed at the intersection oflayers 12 and 14.

Aluminum ohmic contacts 16, 18 and 20 are respectively formed on exposedupper surfaces of layers 12 and 14 and substrate 10. Electrodes 16, 18and 20 are connected to suitable electronic circuits to bias the deviceinto a photoconductive state or enable the device to operate in thephotovoltaic mode. All remaining, exposed surfaces of substrate 10 andof layers 12 and 14 are covered with passivating silicon dioxide (SiOn)layer 20.

The structure of FIG. 1 can be modified to detect infrared energy havinga single cut-off wavelength of 5 . .microns.!. .Iadd.micrometers.Iaddend.or 12 . .microns.!. .Iadd.micrometers.Iaddend.. To provide acut-off wavelength of only 5 . .microns.!. .Iadd.micrometers.Iaddend.,layer 14 and electrode 16 associated therewith are eliminated and thedevice is arranged so the infrared energy is incident on layer 12. Toprovide a cut-off wavelength of 12 . .microns.!..Iadd.micrometers.Iaddend., layer 12 is replaced with a superlatticearrangement of In_(1-x3) Tl_(x3) P (where x₃ varies from 0.67 to 0.57)that is lattice matched to substrate 10.

In an actual staring infrared focal plane detector, many devices of thetype illustrated in FIG. 1 are arranged in a matrix of rows and columnson InP substrate 10 on which are also deposited CMOS bias and readouttransistors, as well as metal row and column strips and othercomponents.

While the preferred configuration includes an InP substrate and one ormore In_(1-x) Tl_(x) P layers, the substrate can also be bulksemimetallic InAs carrying a semiconductor zinc blende layer ofIn_(1-x3) Tl_(x3) As, where 0≦x₃ ≦1; for a 5 . .micron.!..Iadd.micrometers .Iaddend.cut-off of the In_(1-x3) Tl_(x3) As layer, x₃=0.15. The invention is not limited to the pseudobinary compoundsIn_(1-x) Tl_(x1) P and In_(1-x2) Tl_(x2) As for layers 12 and 14, butcan be expanded to include the generalized pseudotertiary compound ..In_(1-x) Tl_(x1) P.!. .Iadd.In_(1-x4) Tl_(x4) Q.Iaddend., where Q isselected from the group consisting essentially of As_(1-y) P_(y), where0<x₄ <1 and 0≦y≦1. For y=0 or y=1, we have the specialized cases of thepseudobinary compounds In_(1-x3) Tl_(x3) As and In_(1-x3) Tl_(x3) P,respectively. For 0<y<1, we have the above-noted generalizedpseudotertiary compound which our studies show can detect and emitinfrared energy to tailored wavelengths in the spectra of interest. Itis to be understood that the substrates are not limited to the preferredcompounds of InP and InAs but that the substrate can be formed of othermaterials, particularly silicon or gallium arsenide in semiconductorform. The In_(1-x) Tl_(x) Q layer carried by such a substrate isphysically connected to the substrate by an appropriate superlatticestructure.

Based on the studies we have performed, the bandgap energies (hence thecut-off wavelengths) of In_(1-x) Tl_(x) P and In_(1-x) Tl_(x) As (shownin FIG. 2 by plotted lines 22 and 24, respectively) in accordance withthis invention and the prior art compound Hg_(x) Cd_(1-x) Te (shown byplotted lines 26) are illustrated as a function of the value of x in theinterval 0≦x≦1. From FIG. 2, any desired band gap, hence cut-offwavelength, can be attained by proper selection of the value of x. Thehigher electron mobilities of the compounds of the present inventionrelative to the mobility of the prior art HgCdTe at temperaturesapproaching room temperature are clearly shown in FIG. 3. FIG. 3includes plots based on our studies of electron mobility (in . .10⁵ cm²/V.sec.!. .Iadd.10⁵ cm² /V·s.Iaddend.) vs. temperature of In₀.33Tl.sub..67 P (line 28), In.sub..85 Tl.sub..13 As (line 30) andHg.sub..78 Cd.sub..22 Te (line 32), all p doped with zinc at 10¹⁴ /cm³to have a band gap energy of 0.1 eV. Hence, our studies show that layersof the present invention do not require the extensive refrigerationstructure required by the prior art.

The ruggedness, i.e. structural stability, of the zinc blende (four-foldcoordination) In_(1-x) Tl_(x) P and In_(1-x) Tl_(x) As latticestructures of the present invention can be determined from the bindingenergy of the atoms of these compounds. Our studies have shown thesecompounds in zinc blende form to be light open structures having strongdirectional bonds relative to other compounds of the cations Al.Iadd.,.Iaddend.Ga and In with the anions P, As and Sb in more closely packedNaCl (six-fold coordination) and CsCl (eight-fold coordination)structures. For TlSb, the NaCl and CsCl structures overtake the zincblende structure, a manifestation of which is a very negative band gapof TlSb. This reversal in the ordering of the TlSb energy causescomplications when attempts are made to grow the prior art In_(1-x)Tl_(x) Sb alloy. Our studies show that TlP and TlAs are stable relativeto the more closely packed phases of the prior art compounds, wherebyIn_(1-x) Tl_(x) P and In_(1-x) Tl_(x) As are stable and can be producedwithout excessive problems.

Our studies show In.sub..33 Tl.sub..67 P has excellent long wavelengthinfrared properties relative to the prior art Hg.sub..78 Cd.sub..22 Tebecause inter alia:

1. its . .5.96 Å.!. .Iadd.596 pm (picometer) .Iaddend.lattice constantclosely matches the . .5.83 Å.!. .Iadd.583 pm (picometer).Iaddend.lattice constant of the InP substrate on which it is depositedso the In.sub..33 Tl.sub..67 P liquidus and solidus phase diagrams havesimple lens shapes;

2. the cohesive energy per atom (2.56 eV/atom) of TlP is 58% greaterthan that of HgTe (1.62 eV/atom);

3. TlP is a semimetal having a band gap of -0.27 eV, about the same asHgTe (-0.3 eV);

4. its band gap concentration variation (dEg/dx) of 1.42 is 16% smallerthan Hg.sub..78 Cd.sub..22 Te;

5. its elastic constants are about 33% greater than those of Hg.sub..78Cd.sub..22 Te;

6. the temperature variation of the band gap (dEg/dT) near . .77°K.!..Iadd.77K .Iaddend.is small (about -0.05 . .meV/°K..!..Iadd.meV/K.Iaddend.), approximately 15% that of Hg.sub..78 Cd.sub..22Te (about -0.36 . .meV/°K..!. .Iadd.meV/K.Iaddend.); because of the lowvalue of dEg/dT for In.sub..33 Tl.sub..67 P, design of circuitsincluding that compound for variable temperature operation is greatlysimplified and spatial variations in pixel performance of detectorelements in a large matrix array due to temperature gradients within thearray are virtually eliminated;

7. its electron effective mass is 0.008, equal virtually to that ofHg.sub..78 Cd.sub..22 Te; and

8. its hole effective mass is 0.37, 43% smaller than the 0.65 holeeffective mass of Hg.sub..78 Cd.sub..22 Te, resulting in considerablyhigher hole mobility and substantially longer electron Augerrecombination lifetimes for In.sub..33 Tl.sub..67 P.

While there have been described and illustrated several specificembodiments of the invention, it will be clear that variations in thedetails of the embodiments specifically illustrated and described may bemade without departing from the true spirit and scope of the inventionas defined in the appended claims. For example, the invention can beused to form a solar cell having a cut-off wavelength such that a verylarge portion of the infrared spectrum is converted by photovoltaicaction into electrical energy; in such an instance a layer of In.sub..24Tl.sub..76 P is formed on an InP substrate. The compounds of theinvention can also be used to form layers of infrared emitters incombination with the usual other structures of such emitters.

We claim:
 1. An infrared detector or emitter device comprising asubstrate, and a semiconductor layer of In_(1-x) Tl_(x) Q carried by thesubstrate, where Q is selected from the group consisting essentially ofAs_(1-y) P_(y) and . .0<x<.!..Iadd.0<x<1.Iaddend., 0≦y≦1.
 2. The deviceof claim 1 where y=1.
 3. The device of claim 2 where x=0.67.
 4. Thedevice of claim 2 where x=0.57.
 5. The device of claim 1 where x=0.24.6. The device of claim 2 wherein the layer is formed on the substrateand the substrate portion on which the layer is formed consistsessentially of InP.
 7. The device of claim 6 wherein the layer is dopedto have a first conductivity polarization.
 8. The device of claim 7wherein another layer of In_(1-x) Tl_(x) P having the secondconductivity polarization is formed on the layer having the firstpolarization to form a p-n homojunction.
 9. The device of claim 1wherein y=0.
 10. The device of claim 9 wherein x=0.15.
 11. The device ofclaim 10 wherein the layer is formed on the substrate and the substrateportion on which the layer is formed consists essentially of InAs. 12.The device of claim 1 wherein the layer is doped to have a firstconductivity polarization, and a second layer having substantially thesame compound as the layer having the first conductivity polarizationcontacting the first conductivity polarization layer to form a p-nhomojunction, the second layer being doped to have a second conductivitypolarization.
 13. The device of claim 1 wherein the layer is formed onthe substrate and the substrate portion on which the layer is formedconsists essentially of InQ, the layer and substrate havingsubstantially the same lattice constants.
 14. The device of claim 13where y=1.
 15. The device of claim 13 where y=0.
 16. The device of claim13 where 0<y<1.
 17. The device of claim 1 where 0<y<1.
 18. The device ofclaim 1 wherein the device is a detector and the substrate includes asecond layer of In_(1-z) Tl_(z) Q, where z is less than x, the secondlayer being positioned above the In_(1-x) Tl_(x) Q layer so certainoptical radiation wavelengths incident on the second layer pass throughthe second layer and are absorbed by the In_(1-x) Tl_(x) Q layer andother optical radiation wavelengths incident on the second layer areabsorbed thereby.
 19. The device of claim 18 where y=1, x=0.67, z=0.57.20. The device of claim 1 where y=1, x=0.24.
 21. The device of claim 1further including an ohmic contact on the layer.
 22. In In_(1-x) Tl_(x)Q, where Q is selected from the group consisting essentially of As_(1-y)P_(y) and 0<x<1, 0≦y≦1.
 23. The composition of claim 22 where y=0. 24.The composition of claim 22 where y=1.
 25. The composition of claim 22where 0<y<1. .Iadd.
 26. A stoichiometric semiconductor compoundcomprising the elements In, Tl, and including at least one of theelements of As and P, wherein the compound is stoichiometric withrespect to at least three of said elements. .Iaddend..Iadd.
 27. Thestoichiometric semiconductor compound of claim 26 wherein the compoundis stoichiometric with respect to four elements including at least threeof said four elements. .Iaddend..Iadd.28. The stoichiometricsemiconductor compound of claim 27 wherein the compound includesdopants. .Iaddend..Iadd.29. The stoichiometric semiconductor compound ofclaim 26 wherein the compound is an alloy including at least three ofsaid elements in a zinc blende structure. .Iaddend..Iadd.30. Thestoichiometric semiconductor compound of claim 26 wherein the compoundis stoichiometric with respect to four elements, including said fourelements. .Iaddend..Iadd.31. An infrared detector or emitter devicecomprising a substrate, and a semiconductor layer carried by thesubstrate, the layer including a stoichiometric semiconductor compoundcomprising the elements In, Tl, and including at least one of theelements of As and P, wherein the compound is stoichiometric withrespect to at least three of said elements carried by the substrate..Iaddend..Iadd.32. The infrared detector or emitter device of claim 31wherein the compound is stoichiometric with respect to four elements,including at least three of said four elements. .Iaddend..Iadd.33. Theinfrared detector or emitter device of claim 32 wherein the compoundincludes dopants. .Iaddend..Iadd. . The infrared detector or emitterdevice of claim 31 wherein the compound is stoichiometric with respectto four elements, including said four elements.